The present invention relates to capacitive pressure transducers. More specifically, the present invention relates to an improved method and apparatus for forming a reference pressure within a chamber of a capacitive pressure transducer assembly.
FIG. 1A depicts a cross-sectional side view of an assembled prior art capacitive pressure transducer assembly 10. FIG. 1B is an exploded view of the upper housing 40, diaphragm 56 and lower housing 60 of FIG. 1A. Briefly, capacitive pressure transducer assembly 10 includes a body that defines an interior cavity. A relatively thin, flexible ceramic diaphragm 56 divides the interior cavity into a first sealed interior chamber 52 and a second sealed interior chamber 54. As will be discussed in greater detail below, diaphragm 56 is mounted so that it flexes, moves, or deforms, in response to pressure differentials in chambers 52 and 54. Transducer assembly 10 provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure between chambers 52 and 54. The parameter provided by transducer assembly 10 indicative of the differential pressure is the electrical capacitance between diaphragm 56 and one or more conductors disposed on an upper housing 40.
Capacitive pressure transducer assembly 10 includes a ceramic upper housing 40 and a ceramic lower housing 60. The upper housing 40, which generally has a circular shape when viewed from the top, defines an upper face 41, a central lower face 47, an annular shoulder 42 that has a lower face 42a and an annular channel 43 that is located between the central lower face 47 and the annular shoulder 42. Lower face 42a of the annular shoulder 42 is substantially co-planar with central lower face 47. The upper housing further defines an aperture (or passageway) 48 that extends through the housing 40 from the upper side to the lower side. A metallic conductor 46 is disposed on a center portion of the lower face 47.
The diaphragm 56 is generally a circular thin diaphragm that has an upper face 57 and an opposite, lower, face 59. A metallic conductor 58 is disposed on a center portion of upper face 57 of the diaphragm 56. The diaphragm 56 and the upper housing 40 are arranged so that the conductor 46 of the upper housing 40 is disposed opposite to the conductor 58 of the diaphragm 56. Diaphragm 56 is coupled to the upper housing 40 by a high-temperature air-tight seal (or joint) 70. The seal 70 is located between the lower face 42a of the annular shoulder 42 of the upper housing 40 and a corresponding annular portion of face 57 of diaphragm 56. When sealed, the upper housing 40, seal 70 and diaphragm 56 define reference chamber 52. A reference pressure is established and maintained in the reference chamber 52. Aperture 48 provides an inlet or entry way into reference chamber 52.
The lower housing 60, which generally has a circular shape, defines a central opening 64 and an upwardly projecting annular shoulder 62 that has an upper face 62a. The upper face 62a of shoulder 62 of the lower housing 60 is coupled to a corresponding portion of lower face 59 of diaphragm 56 by a high-temperature air-tight seal (or joint) 76. Seal 76 can be deposited and fabricated in a manner similar to that of seal 70. When sealed, the lower housing 60, seal 76 and face 59 of the diaphragm 56 define process chamber 54.
A pressure tube 66 having an inlet passageway 68 is coupled to the lower housing 60 by a seal, for example, so that the inlet passageway 68 is aligned with the opening 64 of the lower housing 60. Accordingly, the process chamber 54 is in fluid communication, via opening 64 and inlet passageway 68, with an external environment. In operation, the capacitive pressure transducer assembly 10 measures the pressure of this external environment.
Conductors 46 and 58 of the capacitive pressure transducer assembly 10 form parallel plates of a variable capacitor C. As is well known, C=Aεrε0/d, where C is the capacitance between two parallel plates, A is the common area between the plates, ε0 is the permittivity of a vacuum, εr is the relative permittivity of the material separating the plates (e.g., εr=1 for vacuum), and d is the axial distance between the plates (i.e., the distance between the plates measured along an axis normal to the plates). So, the capacitance provided by capacitor C is a function of the axial distance between conductor 46 and conductor 58. As the diaphragm 56 moves or flexes up and down, in response to changes in the pressure differential between chambers 52 and 54, the capacitance provided by capacitor C also changes. At any instant in time, the capacitance provided by capacitor C is indicative of the instantaneous differential pressure between chambers 52 and 54. Known electrical circuits (e.g., a “tank” circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor C) may be used to measure the capacitance provided by capacitor C and to provide an electrical signal representative of the differential pressure. Conductors 46, 58 can be comprised of a wide variety of conductive materials such as gold or copper, for example, and can be fabricated via known thin and thick film processes or other known fabrication methods. When thin film processes are utilized, conductors 46, 48 may have thicknesses of about 1 μm, for example.
Diaphragm 56 is often made from aluminum oxide. Other ceramic materials, such as ceramic monocrystalline oxide materials, however, may also be used. Capacitance sensors having ceramic components are disclosed in U.S. Pat. Nos. 5,920,015 and 6,122,976.
As noted above, changes in the differential pressure between chambers 52, 54 cause diaphragm 56 to flex thereby changing the gap between conductor 46 and conductor 58. Measurement of changes in the gap permits measurement of the differential pressure. The gap, however, can also be affected by factors unrelated to pressure. For example, the gap can be affected by changes in temperature. Since the components of transducer assembly 10 can be made from a variety of different materials, each of which has its own characteristic coefficient of thermal expansion, temperature changes in the ambient environment can cause the diaphragm 56 to move closer to, or further away from, conductor 46. Fortunately, changes in the gap caused by temperature changes are characteristically different than changes in the gap caused by changes in differential pressure. To compensate for changes in the gap that are caused due to changes in the ambient temperature, it is known to include a second conductor (not shown) that is disposed adjacent to conductor 46 on the lower face 47 of the upper housing 40. In such an embodiment, conductors 46 and 58 form parallel plates of a variable capacitor C1 and conductor 58 and the second conductor form parallel plates of a variable capacitor C2. The two capacitors, C1 and C2, may be used by known methods to reduce the transducer's sensitivity to temperature changes.
The upper housing 40 is positioned so that the lower face 47, and any conductors disposed thereon, are disposed in a plane that is parallel to the plane defined by the conductor 58 (i.e., diaphragm 56) when the pressures in chambers 52, 54 are equal. As discussed above, the capacitance defined by the conductors 46, 58 depends upon the gap (i.e., axial distance) that exists between these opposing conductors. The gap, which is relatively small (e.g., on the order of 0.0004 inches (10-12 μm)), depends, in part, upon the thickness of the seal 70 and the shape and configuration of the upper housing 40 (e.g., the amount that lower face 42a is out of plane, i.e. offset, with lower face 47, if any).
In operation, capacitive pressure transducer assembly 10 is normally used as an absolute pressure transducer. In this form, reference chamber 52 is evacuated to essentially zero pressure, e.g., less than 10−8 Torr, and the reference chamber 52 is then sealed. The reference pressure then serves as a baseline from which a pressure within the process chamber 54 is determined. To maintain the essentially zero pressure within the reference chamber 52, the transducer assembly 10 includes a tube 80, a cover 82, a hold-wire 86, a screen 88 and a getter element 84. As is shown in FIGS. 1A and 1B, the screen 88 supports the getter element 84 within a hollow portion of the tube 80 while the hold-wire 86 maintains the getter element 84 against the screen 88. The hollow portion of the tube 80 is disposed over the aperture 48 of the upper housing 40 so that the getter element 84 is in fluid communication with the reference chamber 52. In addition to supporting the getter element 84, screen 88 also prevents particles from passing into the reference 52 that could adversely affect the operation of the diaphragm 56.
The bottom end of the tube 80 is coupled to the upper face 41 of the upper housing 40 around the aperture 48 by a high-temperature air-tight seal 92, while the cover 82 is coupled to the upper end of the tube 80 by a low-temperature air-tight seal 94. Seals 92, 94 and seal 70, which is located between the shoulder 42 of the upper housing 40 and the diaphragm 56, all assist in maintaining the reference pressure that is established in the reference chamber 52. The high-temperature seal 92 is comprised of a high-temperature glass material while the low-temperature seal 94 is comprised of a low-temperature glass material. To form the high-temperature seal 92, the high-temperature glass material is deposited on the lower end of the tube 80, a corresponding sealing area of face 41, or both. The high-temperature glass material is melted, a force perpendicular to the upper face 41 of the upper housing 40 is applied between the tube 80 and the upper housing 40 and the high-temperature glass material is then allowed to cool (i.e., solidify) thus forming the high-temperature air-tight seal 92. The low-temperature seal 94 is similarly formed between the upper end of the tube 80 and a corresponding sealing area of the cover 82. The high-temperature glass material of the high-temperature seal 92 has a melting temperature that is higher than that of the low-temperature glass material of the low-temperature seal 94. To provide different melting temperatures, the glass materials of the seals 92, 94 can be comprised of different materials or have different amounts of a common material. The melting temperature of the high-temperature seal 92 is higher than the melting temperature(s) of the high-temperature seals 70 and 76 and the melting temperature(s) of the high-temperature seals 70 and 76 is higher than the melting temperature of the low-temperature seal 94.
The getter element 84 is comprised of a material that, when activated, acts to effectively absorb any gaseous impurities that may be present within the sealed reference chamber 52. Thus, when activated, the getter element 84 assists in maintaining the reference pressure at an ultra high vacuum level for long periods of time, e.g., ten or more years.
Although an ultra high vacuum pressure, i.e., essentially zero pressure, is a convenient and useful reference pressure, other reference pressures can also be used. After the reference pressure has been established in chamber 52, the pressure tube 66 is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling the pressure tube 66 in this fashion delivers the fluid, the pressure of which is to be measured, to process chamber 54 (and to the lower face 59 of the diaphragm 56). The center of diaphragm 56 moves or flexes up or down in response to the differential pressure between chamber 52 and 54 thereby changing the capacitance of capacitor C. Since the instantaneous capacitance of capacitor C is indicative of the position of the diaphragm 56, transducer assembly 10 permits measurement of the pressure in chamber 54 relative to the reference pressure that is established in chamber 52.
The accuracy of the capacitive pressure transducer assembly 10 can depend upon the accuracy at which the reference pressure can be established and maintained in the reference chamber 52. In other words, as the actual pressure within the reference chamber 52 deviates from an intended and designed reference pressure, the performance of the capacitive pressure transducer assembly 10 will correspondingly suffer.
The steps of establishing a reference pressure in the reference chamber 52, activating the getter element 84 and sealing the cover 82 to the tube 80 are typically the last few steps that are performed when fabricating capacitive pressure transducer assembly 10. Thus, the steps of coupling the upper housing 40 to the diaphragm 56 via the high-temperature seal 70, coupling the lower housing 60 to the diaphragm 56 via the high-temperature seal 76, coupling the pressure tube 66 to the lower housing 60 around the opening 64, and coupling the tube 80 (having the screen 88, getter element 84 and hold-wire 86) to the face 41 of the upper housing 40 around the aperture 48 via the high-temperature seal 92 will usually have already been completed before the reference pressure is established.
To establish a reference pressure within the reference chamber 52, the reference chamber 52 is typically subjected to a burn-out and evacuation process and then the cover 82 is sealed to the tube 80. The reference chamber 52 is “burned-out” by heating the inner surfaces that define the reference chamber 52 (including the surfaces of the cover 82, tube 80, housing 40 that are in fluid communication with the reference chamber 52), and the chamber 52 is “evacuated” by drawing an ultra-high vacuum on the reference chamber 52. The burn-out heat vaporizes the contaminants, e.g., volatiles, moisture, that may be present on these inner surfaces while the evacuation vacuum draws the vaporized contaminants and gases out of the reference chamber 52. Since the cover 82 has not yet been sealed to the tube 80, the contaminants and gases are sucked out of the reference chamber 52, the aperture 48 and the hollow portion of the tube 80. Once the burn-out and evacuation process is completed and while the vacuum pressure is continuing to be maintained, the cover 82 is then sealed to the tube 80 via the low-temperature seal 94 to establish the reference pressure in the reference chamber 52.
FIGS. 2A and 2B illustrate a prior art method and apparatus that is used to establish a reference pressure within the reference chamber 52 of a capacitive pressure transducer assembly 10. FIG. 2A generally depicts the burn-out and evacuation process while FIG. 2B generally depicts the process by which the cover 82 is sealed onto the upper end of the tube 80. The apparatus includes a vacuum housing 93 that defines an interior vacuum chamber 95. Referring to FIG. 2A, a low-temperature sealing material 94a is deposited on the upper end of the tube 80. The semi-completed transducer assembly 10, i.e., one that does not yet have the cover 82 sealed to the tube 80, is then disposed in the vacuum chamber 95. After the transducer assembly 10 has been placed in the vacuum chamber 95, the vacuum housing 93 is placed in an oven (not shown), a vacuum source (not shown) is coupled to the vacuum chamber 95 and the burn-out and evacuation process of the reference 52 is initiated. During the burn-out and evacuation process, which can last for more than 20 hours, the transducer assembly 10 is heated to a temperature of about 250° C. and an ultra-high vacuum pressure of the order of 10−8 Torr (or less) is generated in the vacuum chamber 95. In FIG. 2A, the burn-out and evacuation of reference chamber 52 (and aperture 48 and tube 80) is indicated by the arrows which extend from the reference chamber 52, up through the aperture 48 and up through and out of the top end of the tube 80.
After the burn-out and evacuation of the reference chamber 52 is completed, the cover 82 is then coupled to the tube 80 by the low-temperature seal 94. Cover 82 is attached and sealed to the tube 80 without opening vacuum housing 93 so as to preserve the vacuum in reference chamber 52. Accordingly, as can been seen in FIG. 2A, prior to initiating the burn-out and evacuation process, the cover 82 is attached to an end of a rod 96 which penetrates into the vacuum chamber 95 of the vacuum housing 93. When the burn-out and evacuation process is completed, the rod 96 can be actuated to bring the cover 82 in contact with the low-temperature sealing material 94a that is disposed on the upper end of the tube 80.
The low-temperature sealing material 94a that forms the low-temperature seal 94 is not melted during the burn-out and evacuation process, i.e., the burn-out temperature is generally set below the melting temperature of the low-temperature sealing material 94a. Moreover, the burn-out and evacuation process should not compromise the seals that have already been formed in the transducer assembly 10 (e.g., high-temperature seals 70, 76 and 92) and, thus, the burn-out temperature should not exceed the melting temperatures of these seals.
A high-temperature dynamic seal 99 (e.g., a gasket) is disposed in the vacuum housing 93 where the rod 96 penetrates the vacuum housing 93. The high-temperature dynamic seal 99 allows to the rod to travel freely up and down while assisting to maintain the pressure that is present in the vacuum chamber 95 of the vacuum housing 93.
Prior to initiating the burn-out and evacuation process, cover 82 is attached to the end of the rod 96 by a low-temperature seal 98. The melting temperature (i.e., melting point) of the low-temperature seal 98, which is lower than the melting temperature of the low-temperature sealing material 94a, is higher than the burn-out temperature and, therefore, does not melt during the burn-out and evacuation process. The rod 96 extends through the high-temperature dynamic seal 99 and, together with the cover 82, is aligned with the tube 80 of the transducer assembly 10.
Referring now to FIG. 2B, after the burn-out and evacuation process is completed, while the pressure in the vacuum chamber 95 is still being maintained, the rod 96/cover 82 is lowered until the cover 82 comes into contact with the low-temperature sealing material 94a. The temperature within the vacuum chamber 95 (as directed by the oven) is then elevated to cause the low-temperature sealing material 94a to melt. This increase in temperature also causes the low-temperature seal 98 to melt and causes the getter element 84 to become activated. To form the low-temperature air-tight seal 94 between the cover 82 and the tube 80, the temperature within the vacuum chamber 95 is decreased until the low-temperature sealing material 94a solidifies and, while the low-temperature seal 98 is sufficiently melted, the rod 96 is pulled away from the transducer assembly 10. Once the low-temperature seal 94 is formed—and the reference pressure in the reference chamber 52 is thus established—the temperature in the vacuum chamber 95 is reduced to ambient temperature, then vacuum source is disconnected and the assembled transducer assembly 10 is removed from the vacuum housing 93.
FIG. 3 illustrates the prior art burn-out, evacuation and sealing process of the apparatus and method of FIGS. 2A and 2B in more detail. In FIG. 3, the x-axis of the process flow represents Time and the y-axis represents Temperature in degrees Celsius. Prior to initiating the burn-out and evacuation process, at Step A of the process flow, the cover 82 is attached to rod 96 via low-temperature seal 98 and the transducer assembly 10, cover 82 and rod 96 are placed in the vacuum chamber 95 (FIG. 2A). During Step A→B, the temperature in the vacuum chamber 95 is raised to a burn-out temperature of 250° C. and the pressure is lowered to an evacuation pressure of 10−8 Torr. Step A→B is completed in three hours. After the burn-out temperature and evacuation pressure are achieved (Step B), the reference chamber 52 is burned-out and evacuated for 20 hours, Step B→C. Shortly before Step C is reached, the rod 96 and cover 82 are lowered so that the cover 82 comes into contact with the low-temperature sealing material 94a that is deposited on the upper end of the tube 80. Once the burn-out and evacuation step is completed (Step C), the temperature in the vacuum chamber 95 is raised to 475° C., Step C→D, which causes the low-temperature sealing material 94a and the low-temperature seal 98 to melt. Step C→D lasts for three hours. The vacuum chamber 95 is then maintained at 475° C. for 30 minutes, Step D→E, to ensure that the low-temperature sealing material 94a and the low-temperature seal 98 are sufficiently melted. The temperature in the vacuum chamber 95 is then lowered to 400° C. over the course of two hours, Step E→F, which causes the low-temperature sealing material 94a to solidify and form the low-temperature air-tight seal 94. The melting temperature of the low-temperature seal 98 is below 400° C. and, thus, the low-temperature seal 98 remains melted throughout Step E→F. Shortly before Step F is reached, rod 96 is raised away from the cover 82 (FIG. 2B). Lastly, the temperature and pressure in the vacuum chamber 95 are brought to ambient conditions over the course of 4 hours and the assembled pressure transducer assembly 10 is then removed from the vacuum chamber 95 of the vacuum housing 93, Step F→G. As illustrated in FIG. 3, the prior art burn-out, evacuation and sealing process can be completed in 32 ½ hours.
The method and apparatus described above does not necessarily ensure that an accurate reference pressure has been established within the reference chamber 52 of a capacitive pressure transducer assembly 10. For example, it is very difficult to establish and maintain an ultra high vacuum of the order of 10−8 Torr (or less) in a vacuum housing 93 that utilizes a rod 96 and a high-temperature dynamic seal 99 because the pressure integrity of the vacuum housing 93 tends to be compromised by the presence of the high-temperature dynamic seal 99. It also can be difficult or costly to accurately control the positions and orientations of the cover 82 and the tube 80 during the rod actuating mating process. If the cover 82 is not positioned or oriented properly in relationship to the tube 80 during the mating process, the integrity of the low-temperature seal 94 may be compromised or the low-temperature seal 94 may fail entirely.
A need therefore exists for a method and apparatus for accurately establishing a reference pressure within a reference chamber of a capacitive pressure transducer assembly.